Corrective optics for reducing fixed pattern noise in head mounted displays

ABSTRACT

An electronic display assembly includes a display element and a corrective element coupled to the display element. The display element has a first plurality of sub-pixels of a first type and a second plurality of sub-pixels of a second type. Two adjacent sub-pixels of the first plurality of sub-pixel are separated by a sub-pixel distance. The corrective element has a plurality of features configured to diffuse light emitted by the first plurality of sub-pixels such that an apparent distance between the two adjacent sub-pixels of the first type, viewed at a viewing distance away from the electronic display assembly, is less than the sub-pixel distance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. application Ser.No. 16/870,545, filed May 8, 2020, which is a continuation of co-pendingU.S. application Ser. No. 15/943,546, filed Apr. 2, 2018, now U.S. Pat.No. 10,684,482, which claims the benefit of U.S. Provisional ApplicationNo. 62/482,126, filed Apr. 5, 2017, which is incorporated by referencein its entirety.

BACKGROUND

The present disclosure generally relates to reducing fixed patternnoise, and specifically relates to reducing fixed pattern noise using atleast a diffractive portion of an optical element.

Electronic displays include a plurality of pixels, which may eachinclude a plurality of sub-pixels (e.g., a red sub-pixel, a greensub-pixel, etc.). Arrangement of individual sub-pixels may affect theappearance and performance of an electronic display device. Somearrangements of sub-pixels may increase fixed pattern noise undercertain conditions. For example, magnification of a pixel may result inboundaries between individual sub-pixels of the pixel becoming visibleto the user, resulting in a “screen door” pattern (i.e., an increase infixed pattern noise) in an image presented to a user.

Prior techniques to reduce the screen door pattern include using amicro-lens array or dual gratings to adjust the image. Dual gratings andmicro-lenses are fabricated using multi-step fabrication processes thatare difficult to control. In addition, micro-lens arrays must be alignedwith sub-pixels to prevent strong moiré patterns from being visible inthe resulting image, which leads to error in micro-lens displays.

SUMMARY

An electronic display assembly presents content to a user. Theelectronic display assembly includes a display element and a correctiveelement coupled to the display element. The display element has a firstplurality of sub-pixels of a first type and a second plurality ofsub-pixels of a second type. Two adjacent sub-pixels of the firstplurality of sub-pixel are separated by a sub-pixel distance. Thecorrective element has a plurality of features configured to diffuselight emitted by the first plurality of sub-pixels such that an apparentdistance between the two adjacent sub-pixels of the first type, viewedat a viewing distance away from the electronic display assembly, is lessthan the sub-pixel distance. The reduction in apparent distance betweenthe two adjacent sub-pixels act to reduce visibility of dark space thatmay otherwise be visible between the two adjacent pixels.

In some embodiments, the electronic display assembly may be a componentof a head-mounted display (e.g., for use in artificial realityapplications). The electronic display assembly may also be a componentof a mobile device (e.g., a mobile phone, tablet, etc.).

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1A is a wire diagram of a head-mounted display (HMD), in accordancewith an embodiment.

FIG. 1B is a cross section of a front rigid body of the HMD shown inFIG. 1A, in accordance with an embodiment.

FIG. 2A is an example array of sub-pixels on an electronic displayelement, in accordance with an embodiment.

FIG. 2B is an example illustrating adjustment of image data of the arrayof FIG. 2A by an optical block, in accordance with an embodiment.

FIG. 3 is an optical block where an optics block is a single opticalelement having a corrective surface, in accordance with an embodiment.

FIG. 4 is a plot of a portion of a design of a sinusoidal grating, inaccordance with an embodiment.

FIG. 5 is a design for a corrective surface having a sinusoidaldiffraction grating overlaid onto a Fresnel lens, in accordance with anembodiment.

FIG. 6A is a plot of a portion of a design of a holographic diffuser, inaccordance with an embodiment.

FIG. 6B is a plot of a 1-dimensional slice of Fourier magnitude at animage plane (less a center point) based on the portion of the design ofthe holographic diffuser in FIG. 6A, in accordance with an embodiment

FIG. 7A is a plot of a portion of a design of a square diffractiongrating, in accordance with an embodiment.

FIG. 7B illustrates plots showing selective blurring (i.e., diffraction)of green light and not red light by a square grating, according to anembodiment.

FIG. 7C illustrates plots showing selective blurring of red light withlittle blurring of green and blue light by a square grating, accordingto an embodiment.

FIG. 8 is an example optical block where an electronic display assemblyincludes a corrective element, in accordance with an embodiment.

FIG. 9A is a plot of a portion of a design of an array of pillars, inaccordance with an embodiment.

FIG. 9B is a plot of Fourier plane intensity based on the design in FIG.9A, in accordance with an embodiment.

FIG. 10A is a plot of a portion of a design of an array of denselypacked pillars, in accordance with an embodiment.

FIG. 10B is a plot of Fourier plane intensity based on the design inFIG. 10A, in accordance with an embodiment.

FIG. 11A is a plot of a portion of a design of an array of steppedpillars, in accordance with an embodiment.

FIG. 11B is a plot of Fourier plane intensity based on the design inFIG. 11A, in accordance with an embodiment.

FIG. 12 is a diagram of a shows a diagram of a sub-pixel light source inwhich the light emitted by the sub-pixel light source is diffused by acorrective element, in accordance with an embodiment.

FIG. 13A is an image of an exemplary pair of sub-pixels, in accordancewith an embodiment.

FIG. 13B shows an exemplary pair of sub-pixels with a diffusion patterncreated by a corrective element, in accordance with an embodiment.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

Embodiments of the invention may include or be implemented inconjunction with an artificial reality system. Artificial reality is aform of reality that has been adjusted in some manner beforepresentation to a user, which may include, e.g., a virtual reality (VR),an augmented reality (AR), a mixed reality (MR), a hybrid reality, orsome combination and/or derivatives thereof. Artificial reality contentmay include completely generated content or generated content combinedwith captured (e.g., real-world) content. The artificial reality contentmay include video, audio, haptic feedback, or some combination thereof,and any of which may be presented in a single channel or in multiplechannels (such as stereo video that produces a three-dimensional effectto the viewer). Additionally, in some embodiments, artificial realitymay also be associated with applications, products, accessories,services, or some combination thereof, that are used to, e.g., createcontent in an artificial reality and/or are otherwise used in (e.g.,perform activities in) an artificial reality. The artificial realitysystem that provides the artificial reality content may be implementedon various platforms, including a head-mounted display (HMD) connectedto a host computer system, a standalone HMD, a mobile device orcomputing system, or any other hardware platform capable of providingartificial reality content to one or more viewers.

FIG. 1A is a wire diagram of a head-mounted display (HMD) 100, inaccordance with an embodiment. The HMD 100 includes a front rigid body105 and a band 110. The front rigid body 105 includes an electronicdisplay element of an electronic display (not shown in FIG. 1A) andoptics block (not shown in FIG. 1A). As discussed in detail the opticsblock and/or the electronic display may include a corrective elementthat reduces fixed pattern noise in the image presented to the user.

In some embodiments, the HMD 100 may act as a virtual reality (VR)headset, an augmented reality (AR) headset, a mixed reality (MR)headset, or some combination thereof. In embodiments that describe ARsystem environment, the HMD 100 augments views of a physical, real-worldenvironment with computer-generated elements (e.g., images, video,sound, etc.). For example, one or more portions the HMD 100 may be atleast partially transparent. In embodiments that describe MR systemenvironment, the HMD 100 merges views of physical, real-word environmentwith virtual environment to produce new environments and visualizationswhere physical and digital objects co-exist and interact in real time.

FIG. 1B is a cross section 115 of the front rigid body 105 shown in FIG.1A, according to an embodiment. As shown in FIG. 1B, the front rigidbody 105 includes an optical block 120, which provides altered imagelight to an exit pupil 125. The exit pupil 125 (also referred to as theeyebox or eyebox region) is the location of the front rigid body 105where a user's eye 130 is positioned. For purposes of illustration, FIG.1B shows a cross section 115 associated with a single eye 130, butanother optical block, separate from the optical block 120, providesaltered image light to another eye of the user.

The optical block 120 includes an electronic display assembly 135 and anoptics block 140. The electronic display assembly 135 projects imagelight towards the corrective optics block 140. As discussed in detailbelow with regard to FIGS. 3-13 , the electronic display assembly 135and/or the optics block 140 can alter the projected image light tocorrect for fixed pattern noise by slightly blurring sub-pixels. Theoptics block 140 directs the altered image light to the exit pupil 125for presentation to the user. In some embodiments, the optics block 118may also magnify and/or correct optical errors (e.g., astigmatism,chromatic aberration, etc.) associated with the projected image.

The electronic display assembly 135 may include one or more electronicdisplay panels. Examples of the electronic display panels include: aliquid crystal display (LCD), an organic light emitting diode (OLED)display, a transparent OLED, an active-matrix organic light-emittingdiode display (AMOLED), some other display, or some combination thereof.

An electronic display panel includes a display area comprising aplurality of pixels and sub-pixels, where a sub-pixel is a discretelight emitting component. A pixel (also referred to as a full pixel)includes a plurality of sub-pixels. In some embodiments, a pixelincludes a plurality of sub-pixels of different colors (e.g., red,green, and blue). Different sub-pixels are separated from each other bydark space. For example, a sub-pixel emits red light, yellow light, bluelight, green light, white light, or any other suitable color of light.In some embodiments, images projected by the electronic display panelare rendered on the sub-pixel level. This is distinct from, say an RGB(red-green-blue) layout, which has discrete red, green, and blue pixels(red, green, and blue) and each pixel in the RGB layout includes a redsub-pixel, which is adjacent to a green sub-pixel that is adjacent to ablue sub-pixel; the red, green, and blue sub-pixels operate together toform different colors. In an RGB layout a sub-pixel in a pixel isrestricted to working within that pixel. However, in some embodiments,sub-pixels in the electronic display operate within multiple “logical”pixels in their surrounding vicinity to form different colors. Thesub-pixels are arranged on the display area of the electronic displaypanel in a sub-pixel array. Examples of a sub-pixel array includePENTILE® RGBG, PENTILE® RGBW, RGB stripes, hex-packed RGBs, and someanother suitable arrangement of sub-pixels that renders images at thesub-pixel level.

In some embodiments, the electronic display assembly 135 may alsoinclude a corrective element (e.g., an array of pillars). As discussedin detail below, the corrective element is coupled to the displaysurface of the electronic display panel and corrects for fixed patternnoise by slightly blurring the image of each sub-pixel so the blurredsub-pixels mask the dark space between the sub-pixels.

The optics block 140 includes one or more optical elements that adjustan image projected by the electronic display assembly 135 to the user bythe HMD 100. In some embodiments, the optics block 140 is positioned atleast 35 mm from the electronic display assembly 135. In someembodiments, at least one surface of an optical element (e.g., a lens)in the optics block 140 includes a corrective surface (e.g., anengineered diffuser, a sinusoidal grating, a holographic diffusor, asquare grating, a trapezoidal grating, etc.). As further discussed inconjunction with FIGS. 3-7C, the corrective element corrects for fixedpattern noise by slightly blurring the image of each sub-pixel so theblurred sub-pixels mask the dark space between the sub-pixels. And insome embodiments, the optics block 140 includes the corrective surfaceand the electronic display assembly 135 includes the corrective element.

The corrective element may be formed using, e.g., features discussed inFIGS. 8-13B. And as discussed below, the corrective element may also beformed using the techniques described in FIGS. 3-7C for creating acorrective surface of an optical element. Likewise, the correctivesurface may be formed using techniques described in FIGS. 8-13B forcreating the corrective element.

Additionally, in some embodiments, an optical element may be anaperture, a filter, or any other suitable optical element that affectsthe image projected by the electronic display 115. In some embodiments,one or more of the optical elements in the optics block 140 may have oneor more coatings, such as anti-reflective coatings.

The optics block 140 magnifies image light projected by the electronicdisplay assembly 135 and corrects optical errors associated with theimage light. Magnification of the image light allows the electronicdisplay assembly 135 to be physically smaller, weigh less, and consumeless power than larger displays. Additionally, magnification mayincrease a field of view of the displayed media. For example, the fieldof view of the displayed media is such that the displayed media ispresented using almost all (e.g., 110 degrees diagonal), and in somecases all, of the user's field of view. However, magnification may causean increase in fixed pattern noise, also referred to as the “screen dooreffect,” which is a visual artifact where dark space separating pixelsand/or sub-pixels of a display become visible to a user in an imagepresented by the display. For example magnification without opticalerror correction may increase fixed pattern noise to the point where theprojected image suffers from the screen door effect. In someembodiments, the optics block 140 is designed so its effective focallength is larger than the spacing to the electronic display assembly135, which magnifies the image light projected by the electronic displayassembly 135. Additionally, in some embodiments, the amount ofmagnification may be adjusted by adding or removing optical elements.

The optics block 140 may be designed to correct one or more types ofoptical error. Optical error may be fixed pattern noise (i.e., thescreen door effect), two dimensional optical errors, three dimensionaloptical errors, or some combination thereof. Two dimensional errors areoptical aberrations that occur in two dimensions. Example types of twodimensional errors include: barrel distortion, pincushion distortion,longitudinal chromatic aberration, transverse chromatic aberration, orany other type of two-dimensional optical error. Three dimensionalerrors are optical errors that occur in three dimensions. Example typesof three dimensional errors include spherical aberration, comaticaberration, field curvature, astigmatism, or any other type ofthree-dimensional optical error. In some embodiments, the media providedto the electronic display 115 for display is pre-distorted, and theoptics block 140 corrects the distortion.

FIG. 2A is an example array 200 of sub-pixels on the electronic displayassembly 135. The example array 200 shown in FIG. 2A includes redsub-pixels 210, blue sub-pixels 220, and green sub-pixels 230. Forexample, the array 200 is portion of a PENTILE® display. In otherembodiments, the array 200 may be in some other configuration (e.g.,RGB).

A dark space 240 separates each sub-pixel from one or more adjacentsub-pixels. The dark space 240 is a portion of the array 200 that doesnot emit light, and may become visible to a user under certaincircumstances (e.g., magnification) causing the screen door effect thatdegrades image quality. As discussed above in conjunction with FIG. 1B,the optics block 140 and/or the electronic display assembly 135 caninclude an optical element (e.g., a corrective surface and/or correctiveelement) configured to reduce fixed pattern noise so the dark space 240between the sub-pixels is not visible to the user (e.g., by blurringeach sub-pixel, creating a blur spot associated with each sub-pixel inthe image). The blur spots are large enough so adjacent blur spots maskthe dark space 240 between adjacent full pixels. In other words, for anydisplay panel, the largest pixel fill-ratio is 100%, if there is no gapat all between sub-pixels. However, to completely get rid of the screendoor artifact on the panel side, the pixel fill-ratio may be muchgreater (e.g., 300%), such that the sub-pixels of different colors areoverlapping. This way, when only green pixels are emitting light, forexample, when viewed with perfect viewing optics, there would be no gapbetween the sub-pixels. This is difficult to do for OLED and/or LCDdisplay panels, but it is doable with a corrective optical element(e.g., a diffractive element, engineered diffuser) as described below.

FIG. 2B is an example illustrating adjustment of image data of the array200 of FIG. 2A by the optical block 120. As shown in FIG. 2B, each ofthe sub-pixels has an associated blur spot. Specifically, the redsub-pixels 210 have a corresponding red blur spot 260, the bluesub-pixels 220 have a corresponding blue blur spot 270, and the greensub-pixels 230 have a corresponding green blur spot 280. A blur spot isan area filled with an image of a blurred sub-pixel. So long as a blurspot does not overlap with a point of maximum intensity of an adjacentblur spot that is created by a sub-pixel of the same color, the two blurspots are resolvable as two adjacent pixels. In some embodiments, thethree sub-pixels all overlap and creates a white pixel. The shape of theblur spot is not necessarily a circle, but is rather an area includingthe blurred image of a sub-pixel. The optics block 140 and/or theelectronic display assembly 135 are configured to blur each sub-pixel sothe blur spots mask the dark space 240 between adjacent pixels. How thisis accomplished is discussed in detail below in conjunction with FIGS.3-13B.

FIG. 3 is an optical block 300 where an optics block 340 is a singleoptical element 310 having a corrective surface 320, in accordance withan embodiment. The optics block 340 may be an embodiment of the opticsblock 140. In the example of FIG. 3 , the optical block 300 includes anelectronic display assembly 335 and the optical element 310. Theelectronic display assembly 335 is an embodiment of the electronicdisplay assembly 135. The optical element 310 includes a correctivesurface 320 and a second surface 330.

The corrective surface 320 is a surface structure that reduces fixedpattern noise in light from the electronic display assembly 335. Thecorrective surface 320 may be, e.g., a sinusoidal grating, a choppedsine function grating, a holographic diffusor, a square grating, atrapezoidal grating, some other surface structure that reduces fixedpattern noise, or some combination thereof. An example sinusoidalgrating is discussed below with regard to FIG. 4 , an exampleholographic diffusor is discussed below with regard to FIGS. 6A-6B, andan example square grating is discussed below with regard to FIGS. 7A-C.Additionally, in some embodiments, the corrective surface 320 may beformed using techniques described for a corrective element below withregard to FIGS. 8-13B. This is discussed in detail below with regard toFIG. 8 . Note that one advantage of having the corrective surface on anoptical element versus, for example, a corrective element coupleddirectly to a display assembly, is that no additional element is needed.An optical element may be injection molded to create the correctivesurface 320, or, e.g., the corrective surface 320 may be diamond turnedinto the optical element 310.

The corrective surface 320 can be designed to make light from the singlepixel appear as an N×n grid (where n and N are both integers), make thesingle pixel appear larger, adjust the shape the pixel appears to be(e.g., make a circular pixel appear rectangular), or some combinationthereof. Additionally, in some embodiments, the corrective surface 320may designed to selectively diffuse light as a function of wavelength.For example, the corrective surface 320 may diffuse red light, but notdiffuse blue light. Additionally, the corrective surface 320 may also bedesigned such that an amount of diffusion varies as a function ofwavelength. For example, the corrective surface 320 may diffuse redlight more than, e.g., green light, and may diffuse green light, morethan, e.g., blue light.

Additionally, in some embodiments, the corrective surface 320 may alsoadjust the optical power of the system. For example, the correctivesurface 320 may be part of a curved surface (e.g., a concave or convexsurface), a Fresnel lens (e.g., as discussed in FIG. 5 ), etc.

In this embodiments, the corrective surface 320 faces to the electronicdisplay assembly 335 and the second surface faces the eye 130. Thisconfiguration may help any pattern on the corrective surface 320 be lessvisible to a user. In alternate embodiments, this orientation isreversed, and the corrective surface 320 faces the eye 130 and thesecond surface 330 faces the electronic display assembly 335.

The second surface 330 may be flat or it may alter image light. Forexample, the second surface 330 may be curved to, e.g., magnify imagespresented by the electronic display 335 and/or correct for opticalerrors in the images presented to the user.

FIG. 4 is a plot 400 of a portion of a design of a sinusoidal grating,in accordance with an embodiment. The sinusoidal grating has peaks, suchas adjacent peaks 410 and 420. In this embodiment, the plot 400 shows apitch of approximately 1 mm between peaks 410 and 420 and other sets ofadjacent peaks, and each peak has a height on the order of 1 micron. Thedesign is such that the sinusoidal grating diffuses light from a singlesub-pixel into a 3×3 grid (centered on the single sub-pixel) of closelyspaced sub-pixels that fill up a dark space that would otherwisesurround the single sub-pixel.

FIG. 5 is a design 500 for a corrective surface having a sinusoidaldiffraction grating overlaid onto a Fresnel lens, in accordance with anembodiment. The corrective surface 500 allows for correction of fixedpattern noise without affecting field curvature. The corrective surface500 includes a series of equally spaced grooves, with the distancebetween the grooves referred to as “pitch width.” The pitch widthdetermines the amount the sub-pixels are blurred. The corrective surface500 diffracts the image light from the electronic display assembly 335,and the diffraction of the image light generates the blur spotsdiscussed above in conjunction with FIG. 2B. In some embodiments, thepitch width has a higher density of grooves near a center of the opticalelement generally corresponding to the area on the user's retinacontaining the highest density of cones.

System parameters may be varied through an optical computer aided design(CAD) program or other suitable method to determine configurations ofthe optical block 300. For example, the optical block 300 has an eyerelief of 10 mm, and the field of view is up to 110 degrees. In otherembodiments, one or more system parameters may be varied; for example,parameters are altered to accommodate a longer eye relief (e.g., 15 mm).

FIG. 6A is a plot 600 of a portion of a design of a holographicdiffuser, in accordance with an embodiment. In the plot 600 theholographic diffuser diffuses light from a red sub-pixel such that itappears larger, e.g., a square that is 80 microns×80 microns. In thisexample, the pixel pitch for the red pixels are 80 microns, so thediffuser increases the size of the red sub-pixel to be large enough suchthat the diffused red sub-pixels fill the gaps between neighboring redsub-pixels. The amount of diffusion can be selected such that a diffusedsub-pixel covers some (or all) of the dark space that would otherwiseappear around the sub-pixel, when only a single color sub-pixel isemitting light. This design provides more blurring to longer wavelengthsthan shorter wavelengths of light, such that green pixels are blurredless than red pixels. Blue pixels are blurred by the smallest amount,and in some cases the blur may not be able to exactly fill the gapbetween blue sub-pixels. However, because the human eye is not verysensitive to blue luminance, and is therefore not sensitive to thescreen door effect in blue light, the smaller amount of blurringtypically is sufficient for suppressing fixed pattern noise in blue. Inthe design 600 the surface height varies over a range of ˜1.6 microns,with an average height of roughly 0.5 microns. The scales for thehorizontal and vertical axes along the diffuser plot are in meters.

One advantage of a holographic diffuser is that moiré patterns are lesslikely to be visible to a user, than, e.g., a corrective surface (orcorrective element) that includes some sort of periodic pattern.

FIG. 6B is a plot 610 of a 1-dimensional slice of Fourier magnitude atan image plane based on the portion of the design of the holographicdiffuser in FIG. 6A, in accordance with an embodiment. This Fourierplane intensity shows that one red sub-pixel (with width of 16 microns)is diffused so that it appears to be a larger sub-pixel of 80 microns inwidth, which is the same as the 80 micron spacing between redsub-pixels. This way, when only red sub-pixels are emitting light, nogaps are seen between adjacent red sub-pixels, and the display screenappears to be uniformly red. In some cases, this may help to reduce thedark spaces between the diffraction orders and further reduce the fixedpattern noise. Also, light leakage into any area outside of the 80×80micron square is minimized, maintaining good display contrast.

FIG. 7A is a plot 700 of a portion of a design of a square diffractiongrating, in accordance with an embodiment. The portion is showing a 10mm×10 mm section of a square diffraction grating. The portion includes aplurality of square diffractive elements that are each approximately 1millimeter in period. The surface height of these square diffractiveelements are −2 microns, 0 microns (flush), or 2 microns. For example,two diffractive elements, element 1 710, and element 2 720, have heightsof 2 microns. Element 1 710 and element 2 720 are separated by theperiod 730, which is approximately one millimeter. A square diffractiongrating is composed of a plurality of cells, such as cell 740. Forexample in FIG. 7A, cell 740 includes 4 squares, each of which is a flatsurface. A square diffraction grating may be manufactured using athree-step microlithography process.

The design of the square diffraction grating diffuses light from asingle pixel into a 3×3 grid (centered on the single pixel) of closelyspaced pixels that fill up a dark space that would otherwise surroundthe single pixel. The amount of diffusion can be selected that the gridsof diffused light covers some (or all) of the dark space that wouldotherwise appear around each pixel, if adjacent sub-pixels of the samecolor are turned on. By choosing different amplitudes for the squaregrating, different wavelengths can be diffused selectively, or havedifferent blur sizes.

For example, FIG. 7B illustrates plots showing selective blurring (i.e.,diffraction) of green light and not red light by a square grating,according to an embodiment. In this example, the amplitude of the 1Dsquare grating is 1.4 microns (2.8 microns for peak to valley), and thedesigned square grating creates 0, +1, −1 orders of diffraction of equalamplitude in green, but it does not diffract in red at all.

FIG. 7C illustrates plots showing selective blurring of red light withlittle blurring of green and blue light by a square grating, accordingto an embodiment. In these plots, the amplitude of the 1D square gratingis 0.85 microns (1.7 microns for peak to valley), or 2 microns (4microns for peak to valley) this grating can create 0, +1, −1 orders ofdiffraction of equal amplitude in red, but it diffracts very little ordoes not diffract in green.

In some embodiments, the 1D grating profiles are stacked on top of eachother to create a 2D grating that can selectively diffract differentwavelengths. Combining different gratings that can selectively diffractdifferent wavelengths provides control over how much blur occurs as afunction of wavelengths.

FIG. 8 is an example optical block 800 where an electronic displayassembly 810 includes a corrective element 820, in accordance with anembodiment. The electronic display assembly 810 is an embodiment of theelectronic display assembly 135.

The electronic display assembly 810 includes an electronic displayelement 830 and the corrective element 820. The corrective element 820is coupled to a display area of the electronic display element 830, suchthat light from pixels/sub-pixels in the electronic display element 830pass through the corrective element 820 before being incident on theoptics block 140. The corrective element 820 is a surface structure thatincludes a plurality of features that reduces fixed pattern noise inlight from the electronic display assembly 810. A feature can be apillar, a stepped pillar, a hole, or a stepped hole. The correctiveelement 820 may be, e.g., a plurality of binary pillars, a plurality ofstepped pillars, a plurality of binary holes, a plurality of steppedholes, 1d or 2d sinusoidal gratings, 1d or 2d square gratings, a choppedsine function grating, a square grating, a trapezoidal grating, gratingswith other shapes, holographic diffuser features, some other surfacestructure that reduces fixed pattern noise, or some combination thereof.Example arrays of binary pillars are discussed below with regard toFIGS. 9A-B and 10A-B, and an example array of stepped pillars isdiscussed below with regard to FIGS. 11A-B. In some embodiments, thefeatures can have arbitrary packing, a square packing, a hexagonalpacking, a semi-random packing, or some combination thereof.

In some embodiments, the corrective element 820 is a film that isapplied (e.g., via an index matched adhesive) to the display area of theelectronic display element 830. In other embodiments, it is directlyetched onto the display area. The features can be etched as a singlestep process, e.g., a single step of etching a series of holes into asurface of the display area, or a single step of etching the regionsaround a series of pillars. As another example, a master can bemanufactured using microlithography, and the master can be used in asingle step nanoimprint lithography process to form the features on asurface of the electronic display element 830. The features of thecorrective element 820 do not need to be aligned to the sub-pixels orfixed to a particular orientation, as in some prior screen doorreduction techniques. The process of manufacturing the correctiveelement 820 is thus simpler, faster, and less prone to errors than priorscreen door reduction processes.

In some embodiments, the corrective element 820 coupled to an electronicdisplay element 830 may be formed using techniques described for acorrective surface on an optical element above with regard to FIGS.3-7C. Likewise, the corrective surface of an optical element may beformed using the techniques described with regard to FIGS. 8-13B. Ineach instance the feature height remains unchanged, and a pitch betweenfeatures is scaled based on the distance from the correctivesurface/corrective element and the electronic display. For example, acorrective surface design for an optical element may be used as acorrective element that is coupled to the electronic display element. Ifa design distance from the corrective element to the electronic displayelement is X, and a design distance from the corrective surface to theelectronic display element is Y a feature pitch is scaled by Y/X toutilize the original design for the corrective surface on a correctiveelement. Similarly, for the opposite case where the corrective elementdesign is to be used as the corrective surface of an optical element,the feature pitch would be scaled by X/Y.

The corrective element 820 can be designed to make light from the singlepixel appear as a plurality of closely spaced pixels that appear atdifferent positions relative to the center pixel (e.g., relative to thecenter pixel the pixel may also appear at a 2 o'clock position, a 4o'clock position, a 6 o'clock position, a 8 o'clock position, a 10o'clock position, and a 12 o'clock position). For example, a hex-packedgeometry of the particular features shown in FIG. 9A, in combinationwith a square sub-pixel, creates a diffraction pattern shown in FIG. 9B,in which the light from the single sub-pixel to appear to come from 7sub-pixels. A different design and/or a different packing geometry givesa different appearance for how a single sub-pixel looks like when seenthrough the corrective element. In some embodiments, the correctiveelement 820 can be designed to make light from the single sub-pixelappear as a plurality of closely spaced sub-pixels such as an N×n grid(where n and N are both integers), make the single sub-pixel appearlarger, adjust the shape the sub-pixel appears to be (e.g., make acircular sub-pixel appear rectangular), or some combination thereof.Additionally, in some embodiments, the corrective element 820 maydesigned to selectively diffuse light as a function of wavelength. Forexample, the corrective element 820 may diffuse red light, but notdiffuse blue light. Additionally, the corrective element 820 may also bedesigned such that an amount of diffusion varies as a function ofwavelength. For example, the corrective element 820 may diffuse redlight more than, e.g., green light, and may diffuse green light, morethan, e.g., blue light.

The average feature pitch (i.e., pillar or stepped pillar, or holes orstepped holes, sinusoidal or square gratings, or other gratings, orholographic diffuser) of the corrective element 820 is smaller than adisplay pixel pitch or sub-pixel pitch. This helps prevent a beatfrequency/moiré pattern occurring which has a same spatial frequency asthe features. If the pitch is too large, one of the artifacts shows upas a beat frequency/moiré pattern that has the same spatial frequency asthe features. The beat frequency/moiré pattern is caused by aninteraction between the “shadows” of the features and the display pixelsor sub-pixels. This pattern is centered along the eye's gaze direction,moves with the eye's gaze and is usually localized within a smallregion. Additionally, to reduce Moiré artifacts, the corrective element820 may have an optimal rotation with regards to the electronic displayelement 830. The distance between the features on the corrective element820 and the display pixels or sub-pixels may also be adjusted tominimize moiré visibility. In some embodiments, for displays with higherthan 400 PPI, this distance is smaller than 1 mm. In some embodiments,the height of pillars and/or holes is between 350 and 800 nm.Additionally, in some embodiments, the heights of other types offeatures is below 10 microns.

The features in the corrective element 820 provide a specificdiffraction pattern on a Fourier plane. The specific diffraction patternexpands a subpixel width to cover dark space that separates adjacentpixels. In one embodiment, the features are design such that a majorityof energy goes equally into the 0, 1, −1 orders, and has minimal leakageinto higher orders, thus providing high contrast and maintaining a smallpitch for the features.

Pitch and fill ratio of the features are related to a pixel geometry andcover glass thickness of the electronic display element 830. They arecalculated using the Fourier transform algorithm. The fill ratio of thefeature is adjusted such that the majority of the energy goes equallyinto the 0, 1, −1 orders, in one embodiment. In other embodiments, thefill ratio and other available parameters are adjusted such that thesub-pixel width is enlarged to the full pixel pitch, when seen throughthe corrective element, and no significant modulation can be resolved byeye in the “enlarged” sub-pixel. The distance between the features tothe display pixels controls the pitch of the features. For example, inone embodiment, the pitch between features is 12 microns, each featurehas a diameter of 7.6 microns, each feature has a height of 0.62microns, and the distance in air between the features and the pixels is325 microns. The distance in air is calculated by dividing themechanical thickness of the film or the coverglass by its refractiveindex. If that distance is scaled, for example by 2 times, than thefeature pitch also is scaled by 2 times. The features may be arranged ina hexagonal packing. The corrective element 820 can be fabricated usingmicrolithography to make the master, and replicated for mass production.

In some embodiments, a corrective element 820 that is a film applied tothe electronic display element 830 may also be used as ananti-fingerprint film, an anti-scratch film, an antiglare film, or somecombination thereof, on a mobile device (e.g., a smart phone, tablet,etc.). Fingerprints are very visible on mobile device displays, becausethe oils in the fingerprint act as microlenses that randomly magnify orotherwise distort individual pixels on the display. Scratches andfeatures on an antiglare film can also act as lenslets and diffractiveoptical features, and magnify or distort individual pixels. When pixelsare magnified or distorted in a non-controlled manner, the defects (suchas finger prints, scratches, or sparkles caused by non-optimal antiglarestructures) are much more apparent to a user. However, using acorrective element (e.g., the corrective element 820) reduces fixedpattern noise such that individual pixels of the electronic displayassembly 810 are not visible, and thus are not individually magnifies ordistorted. Accordingly, a corrective element 820 advantageously makesdefects appear less visible to a user of the mobile device.

FIG. 9A is a plot 900 of a portion of a design of an array of pillarsand/or holes, in accordance with an embodiment. One feature 910 islabelled; the other features are substantially the same. In the plot 900the features are substantially the same height (e.g., about 0.5 microns)and the pitch between features is approximately 10 microns. The fullpixel pitch is about 40 microns, and the width of each sub-pixel isbetween 8 to 16 microns. The cover glass thickness is 300 microns, andthe feature is on top of an adhesive backed plastic film with athickness of 180 microns.

FIG. 9B is a plot 950 of Fourier plane intensity based on the design inFIG. 9A, in accordance with an embodiment. This Fourier plane intensityshows that one sub-pixel appears to be 7 sub-pixels, with the width of afull-pixel, when viewed by the eye through the corrective feature shownin FIG. 9A.

FIG. 10A is a plot 1000 of a portion of a design of an array of denselypacked pillars and/or holes, in accordance with an embodiment. In theplot 1000 the features 1010 (e.g., pillars or holes) are substantiallythe same height (e.g., about 0.5 microns) and the feature pitch 1030between pillars/holes is approximately 12 microns. The fill ratio(feature diameter 1020 divided by feature pitch 1030) is larger thanthat of the pillars/holes in the plot 900, and can further reduce fixedpattern noise. However, in some cases it may cause slight degradation ofcontrast for fine black lines. All other parameters are the same as thedesign shown in FIG. 9A.

FIG. 10B is a plot 1050 of Fourier plane intensity based on the designin FIG. 10A, in accordance with an embodiment. This Fourier planeintensity shows that one sub-pixel appears to be 7 sub-pixels plus a bitmore leakage into the +2 and −2 orders, when viewed by the eye throughthe corrective feature shown in FIG. 10A. The leakage into the higherorders help to further reduce the screen door artifact or fixed patternnoise, but may cause slight degradation of display contrast for finelines.

FIG. 11A is a plot 1100 of a portion of a design of an array of steppedpillars/holes, in accordance with an embodiment. A stepped pillar/holeat its base 1110 has a first diameter, and as the stepped pillar/holemoves away from its base it reaches a step 1120 where the diameter ofthe stepped pillar/hole reduces. In this embodiment, there is a singlestep 1120. But, in other embodiments, additional steps may be included.The addition of steps may increase a number of diffraction orders in thefar field. In this embodiment, a pillar is approximately 2 microns high,with a step 1120 at 0.5 microns, and the design has a feature pitch ofless than 30 microns. All other parameters are the same as the designshown in FIG. 9A.

FIG. 11B is a plot 1150 of Fourier plane intensity based on the designin FIG. 11A, in accordance with an embodiment. This Fourier planeintensity shows that one sub-pixel appears to be 19 sub-pixels, whenviewed by the eye through the corrective feature shown in FIG. 11A. Insome cases, this may help to reduce the dark spaces between thediffraction orders and further reduce the fixed pattern noise.

More intermediate steps can be added to the feature, and in the limit ofan infinite number of steps, the feature becomes a lenslet array.

FIG. 12 is a diagram of a sub-pixel light source 1210 in which the lightemitted by the sub-pixel light source 1210 is diffused by a correctiveelement 1220. The sub-pixel source 1210 is a sub-pixel of the electronicdisplay element 830. The corrective element 1220 is similar to thecorrective element 820 described above, and may be one of the designsshown in FIG. 9A, 10A, or 11A. The features on the corrective element1220 are separated by a pitch, represented as d_(f). When the lightpasses through the corrective element 1220, it splits into multipleimages of the sub-pixel, which may be centered at different orders ofdiffraction; the orders n=±2, n=±1, and n=0 are shown in FIG. 12 . Insome embodiments, such as the example shown in FIG. 9A-B, only ordersn=±1 and n=0 are visible; in other embodiments, additional orders arevisible. The angle between n=0 and n=1 is represented by θ. The displayplane 1230 is the image plane at which a user views the light emitted bythe sub-pixel source 1210. The display plane 1230 and the correctiveelement 1230 are separated by a viewing distance, represented as D. Forexample, the display plane 1230 may be the exit pupil 125 shown in FIG.3 and/or FIG. 8 . The optics block 140, if included, is positionedbetween the corrective element 1220 and the display plane 1230. If theviewing block 140 magnifies or adjusts the image, the viewing distance Dmay represent the apparent distance to the corrective element 1220,rather than the actual distance. In other embodiments, e.g., if thesub-pixel source 1210 is a sub-pixel in a mobile device screen, thedisplay plane 1230 is a position at which a user views the mobiledevice. The pitch between the n=0 and n=1 images on the display plane1230 is represented by y.

FIG. 13A is an image of an exemplary pair of sub-pixels 1300. Twoadjacent sub-pixels 1300 are separated from each other by a pixel pitch,represented as d_(p). Each sub-pixel 1300 is emitted by a respectivesub-pixel source 1210. In this example, the light from the set ofsub-pixels 1300 does not propagate through a corrective element.Accordingly, as described with respect to FIG. 2A, there is a dark spacebetween the two sub-pixels 1300 which may be visible to a user.

FIG. 13B shows the exemplary pair of sub-pixels 1300 with a diffusionpattern created by the corrective element 1220. In the example shown inFIG. 13B, each sub-pixel 1300 is surrounded by six diffused pixel images1350 arranged hexagonally at the n=1 diffraction order around thesub-pixel 1300. As in FIG. 12 , the pitch between the sub-pixel 1300 andeach diffused pixel image 1350 is represented by y. The feature pitchd_(f) that creates the diffusion pattern shown in FIG. 13B can bedetermined for a given pixel pitch d_(p) and viewing distance D.

The dimensions shown in FIG. 12 are related by the following equation:sin θ=y/D  (1)In the example shown in FIGS. 12, 13A, and 13B, the pixel pitch d_(p) isthree times the pixel image pitch y, so equation (1) can be written as:sin θ=(d _(p)/3)/D  (2)Diffraction of light through a grating is related to the wavelength oflight λ emitted by the sub-pixel source 1210 based on the followingequation:d _(f) sin θ=nλ  (3)Using equation (3), equation (2) can be re-written:d _(f)(d _(p)/3)/D=λ  (4)

Solving equation (4) for the feature pitch results in the followingrelationship:d _(f)=3λD/d _(p)  (5)In addition to equation (5), the feature pitch can be furtherconstrained by having the feature pitch d_(f) be smaller than the pixelpitch d_(p) in order to reduce or prevent moiré patterns, as describedabove. Thus, given a pixel pitch d_(p) and viewing distance D, thefeature pitch d_(f) can be selected such that the feature pitch d_(f) isless than the pixel pitch d_(p) and the feature pitch d_(f) satisfiesequation (5).

ADDITIONAL CONFIGURATION INFORMATION

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the disclosure be limited not bythis detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosed embodiments areintended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. An electronic display assembly comprising: adisplay element having a first plurality of sub-pixels of a first typethat is configured to emit light at a first wavelength and a secondplurality of sub-pixels of a second type that is configured to emitlight at a second wavelength, wherein two adjacent sub-pixels of thefirst plurality of sub-pixels are separated by a sub-pixel distance andtwo adjacent sub-pixels of the second plurality of sub-pixels areseparated by a second sub-pixel distance; and a corrective elementcoupled to the display assembly, the corrective element formed from afirst grating profile that is stacked on top of a second gratingprofile, the first grating profile including a plurality of firstfeatures configured to passively diffuse light emitted by the firstplurality of sub-pixels such that an apparent distance between the twoadjacent sub-pixels of the first type, viewed at a viewing distance awayfrom the electronic display assembly, is less than the sub-pixeldistance, and the plurality of first features is also configured topassively diffuse light emitted by the second plurality of sub-pixelsless than the light emitted by the first plurality of sub-pixels, andthe second grating profile including a plurality of second featuresconfigured to passively diffuse light emitted by the second plurality ofsub-pixels such that an apparent distance between the two adjacentsub-pixels of the second type, viewed at the viewing distance away fromthe electronic display assembly, is less than the second sub-pixeldistance, and the plurality of second features is also configured topassively diffuse light emitted by the first plurality of sub-pixels. 2.The electronic display assembly of claim 1, wherein the plurality offirst features are selected from a group comprising one or more holesand one or more pillars.
 3. The electronic display element of claim 1,wherein the wherein the plurality of first features are arranged in asquare packing.
 4. The electronic display element of claim 1, whereinthe wherein the plurality of first features are arranged in a hexagonalpacking.
 5. The electronic display assembly of claim 1, wherein aportion of the sub-pixel distance is dark space, and the plurality offirst features are configured to correct for fixed pattern noise byblurring an image including the two adjacent sub-pixels of the firstplurality of sub-pixels to mask the dark space between the two adjacentsub-pixels of the first plurality of sub-pixels.
 6. The electronicdisplay assembly of claim 1, wherein the plurality of first features areconfigured to blur images of the two adjacent sub-pixels of the firstplurality of sub-pixels by causing light emitted by each sub-pixel toappear as a plurality of closely spaced sub-pixels at the viewingdistance.
 7. The electronic display assembly of claim 1, wherein theplurality of first features is configured to not diffuse light emittedby the second plurality of sub-pixels, and the plurality of secondfeatures is configured to not diffuse light emitted by the firstplurality of sub-pixels.
 8. The electronic display assembly of claim 1,wherein the corrective element comprises a surface of the displayelement, and the plurality of first features is etched onto the surfaceof the display element using a single step process.
 9. The electronicdisplay assembly of claim 8, wherein the plurality of first features isetched onto the surface of the display element using a nanoimprintlithography process.
 10. The electronic display assembly of claim 1,wherein the display element is a screen of a mobile device, and thecorrective element is a film applied to a mobile device to reduce theappearance of defects on the screen.
 11. An electronic display assemblycomprising: a display element having a plurality of sub-pixels of afirst type that is configured to emit light at a first wavelength and asecond plurality of sub-pixels of a second type that is configured toemit light at a second wavelength, wherein two adjacent sub-pixels ofthe first plurality of sub-pixels are separated by a sub-pixel distanceand two adjacent sub-pixels of the second plurality of sub-pixels areseparated by a second sub-pixel distance; and a corrective elementcoupled to the display assembly, the corrective element formed from afirst grating profile that is stacked on top of a second gratingprofile, the first grating profile including a plurality of firstfeatures configured to passively diffuse light emitted by the firstplurality of sub-pixels such that an apparent distance between the twoadjacent sub-pixels of the first type, viewed at a viewing distance awayfrom the electronic display assembly, is less than the sub-pixeldistance, and the plurality of first features is also configured topassively diffuse light emitted by the second plurality of sub-pixelsless than the light emitted by the first plurality of sub-pixels, andthe second grating profile including a plurality of second featuresconfigured to passively diffuse light emitted by the second plurality ofsub-pixels such that an apparent distance between the two adjacentsub-pixels of the second type, viewed at the viewing distance away fromthe electronic display assembly, is less than the second sub-pixeldistance, and the plurality of second features is also configured topassively diffuse light emitted by the first plurality of sub-pixelsless than the light emitted by the second plurality of sub-pixels, andwherein a pitch between adjacent features of the plurality of firstfeatures less is than the sub-pixel distance and the pitch between theadjacent features further satisfies the following equation:d_(f)=3λD/d_(p), wherein d_(f) represents the pitch between adjacentfeatures of the corrective element, λ represents a wavelength of lightemitted by the first plurality of sub-pixels, D represents the viewingdistance, and d_(p) represents the sub-pixel distance.
 12. Theelectronic display assembly of claim 11, wherein the first plurality offeatures are selected from a group comprising one or more holes and oneor more pillars.
 13. The electronic display assembly of claim 11,wherein a portion of the sub-pixel distance is dark space, and theplurality of first features are configured to correct for fixed patternnoise by blurring an image including the two adjacent sub-pixels of thefirst plurality of sub-pixels to mask the dark space between the twoadjacent sub-pixels of the first plurality of sub-pixels.
 14. Theelectronic display assembly of claim 11, wherein the plurality of firstfeatures are configured to blur images of the two adjacent sub-pixels ofthe first plurality of sub-pixels by causing light emitted by eachsub-pixel to appear as a plurality of closely spaced sub-pixels at theviewing distance.
 15. The electronic display assembly of claim 11,wherein the corrective element comprises a surface of the displayelement, and the plurality of first features are etched onto the surfaceof the display element using a single step nanoprint lithographyprocess.
 16. The electronic display assembly of claim 11, wherein theplurality of first features is arranged in a square packing.
 17. Theelectronic display assembly of claim 11, wherein the plurality offeatures is arranged in a hexagonal packing.
 18. The electronic displayassembly of claim 11, wherein the plurality of first features isconfigured to not diffuse light emitted by the second plurality ofsub-pixels, and the plurality of second features is configured to notdiffuse light emitted by the first plurality of sub-pixels.